Method of forming capillary tubing



Patented Oct. 20, 1931 f UNITED STATES PATENT OFFICE DUBWOOD D. BOBENBUBGH, Ol' ROCHESTER, NEW YORK, .ASBIGNOR TO TAYLOR IN- STRUMENT COMPANIES, OF ROCHESTER, NEW YORK, A CORPORATION OF NEW YORK METHOD OF FOBHING CAPILLABY TUBING Original application iiled October i9, 1920, Serial lo. 148,632. Divided and this application illcd July 7,

i 1928. Serial No. 291,111.

This invention relates to instruments including a capillary fluid transmission system and more particularly to apparatus of theA type disclosed in the patent to Hodgkinson No. 1,063 349 dated June 3, 1913.

It has been generally recognized as a desideratum to reduce the size of the capillary for transmitting the fluid pressure from the bulb to the pressure' element so far as this 1s compatible with the pressure requirements at the latter.

Aside from the point of cost, there is another even more cogent reason for reducing the volume of the transmitting liquid, or mercury in particular, to a practical mimmum: Mercury, for instance, has a high coeicient of expansion. Changes in the temperature surrounding the transmission system cause a change in the volume of the transmitting liquid, with the result that this expansion or contraction, as the case may be, reacts upon the pressure condition in the pressurere onsive element, which pressure condition s ould be and is intended to be solely a function of the change in the volume of the liquid in the bulb in response to changes of the temperature conditions to which the bulb is subjected. The greater the volume of the transmitting liquid, the greater is the inluence of changes in its temperature upon the indication ofthe instrument. In all cases where the transmission system is subjected to material variations in temperature, it is necessary to compensate for the changes in the volume of the transmitting iluid caused thereby.

Compensation may be obtained to a certain extent by using a filler wire, such as disclosed in the patent mentioned, of a material having a smaller co-eicient of expansion than the material of the capillary tube. Due to the differential expansion and contraction the variable interspace between the wire core and the walls of the tube thus may have a compensatory effect for the variations in the volume of the transmitting liquid. However, the compensation aiorded is, in the usual case, not entirely satisfactory.

It is the primary object of this invention to provide a simple transmission system which mainaiford complete compensation.

other more general 'object is to produce a transmission conduit having a substantially uniform cross-sectional area.

Still another'particular object is a` method of forming a capilla transmission system of a substantially unilorm and smaller effectig cross-section than was heretofore poss1 e.

Up to the present time it was the general practice, with s Stems having a filler core, to obtain a capi lary having as small a bore as possible and then feeding as large a core as possible into it. A tube havin a bore about .020 diameter together wit a wire having a diameter of about .012 may be considered as typical in the prior art. The clearance is about .004.

The bores of capillaries, as a general rule, are comparatively irregular. The diameter as a whole varies within Wide limits and there are many deformations, such as furrows, depressions and other cavities, all tending to increase the interspace and thereby the volume of the transmitting liquid. In addition to these cavities which directly increase the interspace, there are rough spots distributed along the walls of the capillary, all having the tendency to impede or obstruct the pas sage of the wine through it. The largest wire which may be fed into any tube is considerably smaller in diameter than the bore. It is thus practically impossible to enter a filler core' of less than .006 difference in diameter.

All these inconveniences and disadvantages may be overcome by my invention, which, briefly expressed, consists in drawing or swaging a tube down upon a core to any desired extent.

For a full understanding of the invention, reference is made to the accompanying drawings in which Fig. lis a diagrammatic view of an instrument embodying the invention;

Fig. 2 is a sectional view on a relatively large scale, of a tube and a wire in assembled relation prior to the drawingor swaging operation;

thFi 3 is a similar View of tire same fter e aw or swa n o ra ion; an

Fi 4ubgnd 5 areglvigewsesimilar to Fi 2 an 3, showing a modified form of the 1nvention.

In the drawings 1 represents a temperature-responsive bulb such as is generally used in connection with indicating, regulating and recording instruments of the class involved. The Bourdon tube 2 represents, in a general wa all forms of pressure-responsive devices adiapted to operate under iiu1d pressure and 3 is a capillary transmission system.

In practice I select a wire 3a, which has substantially the desired diameter and then enter it into a tube 3b suiiiciently larger in bore so as to readily pass into and throu h it. In fact, the clearance between the Wa lsof the capillary and the core may be considerably larger than the usual tolerance, be-

causethe tube is to be reduced in size anyway. However, as a matter of common sense, in order to avoid all unnecessary waste of energy and expense, it is preferable to use a wire which just readily enters the tube or a tube which just slips over a core. Inl this way the operation of entering the wireinto the tube does not consume much time and at the same time, the amount of drawing or swaging required is reduced to a relatively small amount.

With the wire in position in the tube, the latter is drawn or swaged upon it. The drawing or swagin operation may beconducted and controlle to obtain any desired de ree of lit, as will be readily understood. owever, so far as the partlcular purpose is concerned, it is preferably so conducted as; to leave just enough clearance to allow a. lm of mercury to form therein. In practice, I find that a good fit is obtained when it requires a considerable force to move the wire by ulling. So long as the wire can be move the interspace is ordinarily not too small. The caplllary mag' be tested by forcing air through it un er adeinite pressure, say about 4 pounds per square inch, into a body of water, and counting bubbles er unit of time. The proper setting of the rawing or swaging may be empirlcally determined for any size of tube and wire correlated therewith and the clearance or interspace may be varied for I different practical requirements, having reference to the pressure conditions desired at the pressure-responsive element, i. e., the Bourdon tube or the like. In other words, the allowable pressure gradient between the two ends of the transmissionv line may vary between certain limits, dependin also to some extent upon the length of t e line.

By measurin the volume of liquid in a capillar of a given length, pre ared according to t is invention, I found t at the clearance may be made as small as .0004 and even .0002. Clearance lower than .0002 are, so

4to a relatively smooth clearance space al minimum for each particular set of conditions, the volume of the transmission liquid is reduced to a practical minimum and the extent of compensation called for is a minimum. This 1s in contradistinction to the prior art which had to accept as the minimum clearance or interspace that which was formed by entering as a large a wire as was possible.

Not only is the interspace reduced in a general way, but the irregularities in the bore, the furrows, depressions and cavities which are present in tubes as unavoidable incidents are reduced in size and extent or even more or less eliminated. Especiall the swaging operation has a peculiar equa izing eiect in this respect.

The result of the drawing or swaging operation, aside from the net reduction in diameter, is the more particular and important leveling or smoothing out of the rough, irregular surface of the bore and the elimination of a previously unavoidable space, which from the standpoint of operation was unnecessary and merely required more compensation than would have been necessary without it. f

It is needless to emphasize that a relatively smooth interspace makes it possible to Obtaln a minimum of resistance to iow for a minimum of volume of fluid. This translated into vpractical parlance, means that it is possible to obtain a fast `instrument with a minimum of transmission iuid.

vWhenever a very fast instrument 1s desired, the resistance flow may be precisely determined by the provision of a groove 4b, in the surface of the core 4a, as indicated in Figs. 4 and 5. The groove may vary from a line thin capillary thread to a groove of more substantial size depending upon the speed desired. The tube 1s drawn or swaged upon the core as before to eliminatel substantially all the waste space and reduce the interspace ing)a film to form.

epending on the volume of mercury, the diameter of the core and the bore may be increased to obtain the dilerential movement necessary for the accommodation of the expanding mercury. The higher the speed i. e. the quicker the response desired, the greater must be the cross-section of the transmission conduit, and the greater .is consequently the volume of mercury. For this vreason the differential movement must be core may have a diameter of .060.065, or even higher. v

At any rate, by providing a groove in the wire core, it is possible to precisely determine the speed of the instrument with a minimum of volume of mercury.

Instead of providing the groove in the surface of the oore, it may be formed in the bore of the tube during the process of drawing. However, I prefer to score or groove the wire core, as shown.

The invention not only has the advantages described, but also the outstanding commercial advantage that it is no longer necessary to use tubes of an extremely small bore with or without Wire filler. Tubes with larger bore, which are easier to make and are less expensive, are now available for the purpose described.

It is understood that in place o a'lsolid core a tubular core may be used. However, for practical purposes a solid core is preferable.

This case is a division from my application Ser. No. 142632 filed Oct. 19, 1926.

I claim: The method of forming capilla tubing for Huid transmission systems in thermometers and the like, which consists in drawing a capillary tubing ofapproximately the de sired dimensions and then swaging the tubing upon a wire to make the bore uniformly smooth and uniform in crossrdsection throughout its length, conducting the swaging operation so that the wire may be withdrawn from the tubing.

In testimon DURYVy ature. v GH.

whereof I aiiix my si OOD D. ROSENB 

